1. Field of the Invention
The present invention relates to a workpiece holding device for a bonding apparatus which holds a workpiece such as a lead frame, film or substrate, etc. in place by pressing such a workpiece against a heat block by means of a workpiece retainer.
2. Prior Art
Generally, in a bonding apparatus, a workpiece is held in place by being pressed against a heat block by a workpiece retainer, and bonding such as die bonding, wire bonding, etc. is then performed on the workpiece. However, the workpiece is heated by a heat bloc; and as a result, it may undergo thermal deformation such as warping, bending, etc.
Japanese Utility Model Application Laid-Open (Kokai) No. S60-192444 (hereafter referred to as Example 1) and Japanese Utility Model Application Laid-Open (Kokai) No. 2-129729 (hereafter referred to as Example 2) disclose bonding apparatuses equipped with means for correcting thermal deformation of the workpiece. In these bonding apparatuses, the workpiece retainer that presses the workpiece against the heat block is equipped with a workpiece pressing means that is driven toward the workpiece so that the driving means is pressed against the workpiece.
In Example 1, two retaining portions are respectively disposed in each of the peripheral edges of the workpiece retainer on two opposite sides of a bonding window (through-hole) of the retainer. Each of the retaining portions has a plate spring which is installed horizontally so as to protrude into the interior of the bonding window, and a claw body with a tapered portion which is superimposed on the plate spring and fastened to the workpiece retainer. The plate spring bends upward in an elastic manner.
In Example 2, numerous retaining pins are installed to protrude from the workpiece retainer (frame body) via plate springs in positions near the periphery of the workpiece so that these pins can advance and retract.
In Examples 1 and 2, the spring plates (Example 1) or retaining pins (Example 2) must be sufficiently raised so that these parts do not contact the workpiece (which has undergone thermal deformation as a result of being heated) during the conveying of the workpiece. If the amount by which these parts are raised is excessively large, the vertical stroke loss is high, and productivity drops. Accordingly, in order to minimize the vertical stroke, the amount of thermal deformation of the workpiece must be checked at the time of product changeover, and the vertical stroke of the workpiece retainer must be adjusted on a case-by-case basis. Considerable time is, however, required for this adjustment.
Furthermore, in Example 1, when the workpiece is lifted by the heat block, the plate springs contact the peripheral edges of the workpiece, and the workpiece is clamped by the heat block and the plate springs. As a result, only the portions of the workpiece where the plate springs are present are pressed against the heat block. In other words, not the entire periphery of the workpiece come into contact with the heat block along the bonding window of the workpiece retainer. In Example 2, the portions of the workpiece that are pressed are the portions where the retaining pins are present. Accordingly, numerous retaining pins and plate springs are required in order to cause the entire periphery of the workpiece to come into contact tightly with the heat block along the bonding window of the workpiece retainer, so that the number of parts required is high, thus causing the apparatus to be expensive.
Accordingly, one object of the present invention is to provide a workpiece holding device for a bonding apparatus in which the vertical stroke of a workpiece retainer can be set in accordance with the type of the workpiece to be processed, thus allowing high-speed operation and being able to respond to thermal deformation of the workpiece.
Another object of the present invention is to provide a workpiece holding device for a bonding apparatus in which, with a small number of elements, thermal deformation of the workpiece is corrected, and the entire periphery of the workpiece is brought to contact the heat block along the bonding window of the workpiece retainer.
The above objects are accomplished by a unique structure for a workpiece holding device used in a bonding apparatus that includes a heat block, a workpiece retainer for pressing a workpiece against the heat block, and a workpiece pressing means which is provided in the workpiece retainer and driven toward workpiece so as to be pressed against the workpiece; and in the present invention:
before the workpiece is pressed by the workpiece pressing means, end portions of the workpiece pressing means which face the workpiece protrude further toward the workpiece than a retaining surface of the workpiece retainer,
when the workpiece is being pressed by the workpiece retainer, the workpiece pressing means undergoes elastic deformation and is pulled in a direction opposite from the workpiece so that the workpiece is pressed against the heat block by the retaining surface of the workpiece retainer, and
when the workpiece is being conveyed, the retaining surface of the workpiece retainer is separated from the workpiece by a predetermined distance so that the workpiece pressing means undergoes elastic recovery, thus preventing the workpiece from contacting the workpiece retainer.
In the above structure, the workpiece pressing means is formed of an elastic material.
Also, the tip end of the elastic material is comprised of a member that can slip easily or has a shape that allows easy slipping.
The workpiece pressing means can be comprised of a pressing member provided in the workpiece retainer and a roller provided at one end of the pressing member so that the roller is rotatable in a direction in which the workpiece is conveyed.